#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main() 
{
    int m = 0;
    int num = 1;
    int joey[3] = { 1 };
    int adult = 0;
    int increase = 0;
    scanf("%d", &m);
    int i = 0;
    for (i = 2; i <= m; i++) 
    {
        adult += joey[1];
        joey[1] = joey[0];
        joey[0] = adult;
        num += adult;
    }
    printf("%d", num);
    return 0;
}